Lignin, a highly branched natural organic polymer which gives structural support to plants, is an underutilized biomaterial that currently enjoys few applications other than as a low-grade fuel for generating heat in pulping processes. After cellulose, lignin is the most abundant renewable carbon source; it has been estimated that between 40 and 60 million tons are produced annually, mostly as a non-commercialized waste product. Interestingly, lignin's complex structure, which is composed of many linked aromatic groups, holds potential as a renewable feedstock for a variety of industrial chemicals and liquid fuels. From highly functionalized monomeric or oligomeric aromatic products, which can be used as starting materials for pharmaceutical and chemical processes, to liquid fuels, lignin's oxidation valorization is potentially wide-ranging. The abundance of lignin as the primary waste from biorefineries and the kraft pulping process means that it could potentially replace a significant amount of fossil raw materials, such as petroleum, as a renewable feedstock for a host of chemicals, with substituted aromatics among the most interesting.
Several methods have been examined to convert lignin to more useful chemicals, including gasification in supercritical water, catalytic steam reforming at high temperatures, oxidative pyrolysis, catalytic hydrodeoxygenation and hydrocracking, hydrothermal fragmentation and condensation, and gasification in aqueous solutions over Pt catalysts. In general, though, such processes have not been able to overcome limitations such as achieving selective oxidation and generation of solid residue like char. Such catalytic processes are often difficult to control, and commercial viability has not been demonstrated.
Electrochemical conversion of waste lignin from pulping mills and biorefineries represents a potentially renewable process for generating industrial chemicals and drop-in replacement fuels without the need for petroleum or other fossil resources. In addition, electrochemical conversion of lignin is potentially more environmentally friendly than some other processes because conversion takes place in moderately alkaline solutions at low to moderate temperature and pressure and electrons driving the reaction can be described as non-polluting reagents. Accordingly, there is a need for an efficient method of converting lignin that addresses the above drawbacks.
Prior attempts to electro-oxidize lignin are not particularly suited for commercialization. In many cases, the catalysts are not efficient or are too expensive. Also, many catalysts are quickly poisoned.